docs: def exist_node(self, graph, node, branch, turn, tick, extant):
          """Declare that the node exists or doesn't.
  
          Inserts a new record or updates an old one, as needed.
  
          """
          if (branch, turn, tick) in self._btts:
              raise TimeError
          self._btts.add((branch, turn, tick))
          self._nodes2set.append((self.pack(graph), self.pack(node), branch, turn, tick, extant))